Liquid-crystalline medium, method for the stabilization thereof, and liquid-crystal display

ABSTRACT

The invention relates to compounds of the formula I, and to a liquid-crystalline medium, preferably having a nematic phase and negative dielectric anisotropy, which comprises
         a) one or more compounds of the formula I       

     
       
         
         
             
             
         
       
         
         
           
             and 
             b) one or more compounds of the formula III-O 
           
         
       
    
                         
in which the parameters have the respective meanings indicated in Claim  1 , to the use thereof in an electro-optical display, particularly in an active-matrix display based on the VA, ECB, PALC, FFS or IPS effect, to displays of this type which contain a liquid-crystalline medium of this type, and to the use of the compounds of the formula I for the stabilization of a liquid-crystalline medium which comprises one or more compounds of the formula III-O.

The present invention relates to novel compounds, in particular for usein liquid-crystal media, but also to the use of these liquid-crystalmedia in liquid-crystal displays, and to these liquid-crystal displays,particularly liquid-crystal displays which use the ECB (electricallycontrolled birefringence) effect with dielectrically negative liquidcrystals in a homeotropic initial alignment. The liquid-crystal mediaaccording to the invention are distinguished by a particularly shortresponse time in the displays according to the invention at the sametime as a high voltage holding ratio (VHR or also just HR for short).

The principle of electrically controlled birefringence, the ECB effector DAP (deformation of aligned phases) effect, was described for thefirst time in 1971 (M. F. Schieckel and K. Fahrenschon, “Deformation ofnematic liquid crystals with vertical orientation in electrical fields”,Appl. Phys. Lett. 19 (1971), 3912). Papers by J. F. Kahn (Appl. Phys.Lett. 20 (1972), 1193) and G. Labrunie and J. Robert (J. Appl. Phys. 44(1973), 4869) followed.

The papers by J. Robert and F. Clerc (SID 80 Digest Techn. Papers(1980), 30), J. Duchene (Displays 7 (1986), 3) and H. Schad (SID 82Digest Techn. Papers (1982), 244) have shown that liquid-crystallinephases must have high values for the ratio between the elastic constantsK₃/K₁, high values for the optical anisotropy Δn and values for thedielectric anisotropy Δ∈ of −0.5 in order to be suitable for use forhigh-information display elements based on the ECB effect.Electro-optical display elements based on the ECB effect have ahomeotropic edge alignment (VA technology=vertically aligned).Dielectrically negative liquid-crystal media can also be used indisplays which use the so-called IPS (In-plane switching) effect.

Industrial application of this effect in electro-optical displayelements requires LC phases which have to meet a multiplicity ofrequirements. Particularly important here are chemical resistance tomoisture, air and physical influences, such as heat, radiation in theinfrared, visible and ultraviolet regions, and direct and alternatingelectric fields.

Furthermore, LC phases which can be used industrially are required tohave a liquid-crystalline mesophase in a suitable temperature range andlow viscosity.

None of the series of compounds having a liquid-crystalline mesophasethat have been disclosed hitherto includes a single compound which meetsall these requirements. Mixtures of two to 25, preferably three to 18,compounds are therefore generally prepared in order to obtain substanceswhich can be used as LC phases.

Matrix liquid-crystal displays (MLC displays) are known. Non-linearelements which can be used for individual switching of the individualpixels are, for example, active elements (i.e. transistors). The term“active matrix” is then used, where in general use is made of thin-filmtransistors (TFTs), which are generally arranged on a glass plate assubstrate.

A distinction is made between two technologies: TFTs comprising compoundsemiconductors, such as, for example, CdSe, or TFTs based onpolycrystalline and, inter alia, amorphous silicon. The lattertechnology currently has the greatest commercial importance worldwide.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully color-capabledisplays, in which a mosaic of red, green and blue filters is arrangedin such a way that a filter element is located opposite each switchablepixel.

The TFT displays most used hitherto usually operate with crossedpolarizers in transmission and are backlit. For TV applications, IPScells or ECB (or VAN) cells are used, whereas monitors usually use IPScells or TN (twisted nematic) cells, and notebooks, laptops and mobileapplications usually use TN cells.

The term MLC displays here encompasses any matrix display havingintegrated non-linear elements, i.e., besides the active matrix, alsodisplays with passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications,monitors and notebooks or for displays with a high information density,for example in automobile manufacture or aircraft construction. Besidesproblems regarding the angle dependence of the contrast and the responsetimes, difficulties also arise in MLC displays due to insufficientlyhigh specific resistance of the liquid-crystal mixtures [TOGASHI, S.,SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E.,WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A210-288 Matrix LCD Controlled by Double Stage Diode Rings, pp. 141 ff.,Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of ThinFilm Transistors for Matrix Addressing of Television Liquid CrystalDisplays, pp. 145 ff., Paris]. With decreasing resistance, the contrastof an MLC display deteriorates. Since the specific resistance of theliquid-crystal mixture generally drops over the life of an MLC displayowing to interaction with the inside surfaces of the display, a high(initial) resistance is very important for displays that have to haveacceptable resistance values over a long operating period.

Displays which use the ECB effect have become established as so-calledVAN (vertically aligned nematic) displays, besides IPS displays (forexample: Yeo, S. D., Paper 15.3: “An LC Display for the TV Application”,SID 2004 International Symposium, Digest of Technical Papers, XXXV, BookII, pp. 758 and 759) and the long-known TN displays, as one of the threemore recent types of liquid-crystal display that are currently the mostimportant, in particular for television applications.

The most important designs may be mentioned here: MVA (multi-domainvertical alignment, for example: Yoshide, H. et al., Paper 3.1: “MVA LCDfor Notebook or Mobile PCs . . . . ”, SID 2004 International Symposium,Digest of Technical Papers, XXXV, Book I, pp. 6 to 9, and Liu, C. T. etal., Paper 15.1: “A 46-inch TFT-LCD HDTV Technology . . . . ”, SID 2004International Symposium, Digest of Technical Papers, XXXV, Book II, pp.750 to 753), PVA (patterned vertical alignment, for example: Kim, SangSoo, Paper 15.4: “Super PVA Sets New State-of-the-Art for LCD-TV”, SID2004 International Symposium, Digest of Technical Papers, XXXV, Book II,pp. 760 to 763) and ASV (advanced super view, for example: Shigeta,Mitzuhiro and Fukuoka, Hirofumi, Paper 15.2: “Development of HighQuality LCDTV”, SID 2004 International Symposium, Digest of TechnicalPapers, XXXV, Book II, pp. 754 to 757).

In general form, the technologies are compared, for example, in Souk,Jun, SID Seminar 2004, Seminar M-6: “Recent Advances in LCD Technology”,Seminar Lecture Notes, M-6/1 to M-6/26, and Miller, Ian, SID Seminar2004, Seminar M-7: “LCD-Television”, Seminar Lecture Notes, M-7/1 toM-7/32. Although the response times of modern ECB displays have alreadybeen significantly improved by addressing methods with overdrive, forexample: Kim, Hyeon Kyeong et al., Paper 9.1: “A 57-in. Wide UXGATFT-LCD for HDTV Application”, SID 2004 International Symposium, Digestof Technical Papers, XXXV, Book I, pp. 106 to 109, the achievement ofvideo-compatible response times, in particular in the switching of greyshades, is still a problem which has not yet been solved to asatisfactory extent.

ECB displays, like ASV displays, use liquid-crystalline media havingnegative dielectric anisotropy (Δ∈), whereas TN and to date allconventional IPS displays use liquid-crystalline media having positivedielectric anisotropy.

In liquid-crystal displays of this type, the liquid crystals are used asdielectrics, whose optical properties change reversibly on applicationof an electrical voltage.

Since in displays in general, i.e. also in displays in accordance withthese mentioned effects, the operating voltage should be as low aspossible, use is made of liquid-crystal media which are generallypredominantly composed of liquid-crystal compounds, all of which havethe same sign of the dielectric anisotropy and have the highest possiblevalue of the dielectric anisotropy. In general, at most relatively smallproportions of neutral compounds and if possible no compounds having asign of the dielectric anisotropy which is opposite to that of themedium are employed. In the case of liquid-crystal media having negativedielectric anisotropy for ECB displays, predominantly compounds havingnegative dielectric anisotropy are thus employed. The liquid-crystalmedia employed generally consist predominantly and usually evenessentially of liquid-crystal compounds having negative dielectricanisotropy.

In the media used in accordance with the present application, at mostsignificant amounts of dielectrically neutral liquid-crystal compoundsand generally only very small amounts of dielectrically positivecompounds or even none at all are typically employed, since in generalthe liquid-crystal displays are intended to have the lowest possibleaddressing voltages.

JP 2006-37054 (A) discloses liquid-crystal mixtures which comprise thethree compounds of the formula

-   -   in which R denotes C₂H₅, C₃H₇ or C₄H₉,        and additionally one or more compounds of the formula

-   -   in which (R,R′) denotes (C₃H₇,C₂H₅), (C₅H₁₁,CH₃) or        (C₅H₁₁,C₂H₅).        For many practical applications in liquid-crystal displays,        however, the known liquid-crystal media are not sufficiently        stable. In particular, their stability to irradiation with UV,        but also even with conventional backlighting, results in an        impairment, in particular, of the electrical properties. Thus,        for example, the conductivity increases significantly.

The use of so-called “hindered amine light stabilizers”, HALS for short,has already been proposed for the stabilization of liquid-crystalmixtures.

Nematic liquid-crystal mixtures having negative dielectric anisotropywhich comprise a small amount of TINUVIN® 770, a compound of the formula

as stabilizer, are proposed, for example, in WO 2009/129911 A1. However,the corresponding liquid-crystal mixtures do not have adequateproperties for some practical applications. Inter alia, they are notsufficiently stable to irradiation using typical CCFL (cold cathodefluorescent lamp) backlighting.

Similar liquid-crystal mixtures are also known, for example, from EP 2182 046 A1, WO 2008/009417 A1, WO 2009/021671 A1 and WO 2009/115186 A1.However, the use of stabilizers is not indicated therein.

According to the disclosure therein, these liquid-crystal mixtures mayoptionally also comprise stabilizers of various types, such as, forexample, phenols and sterically hindered amines (hindered amine lightstabilizers, HALS for short). However, these liquid-crystal mixtures arecharacterised by relatively high threshold voltages and by at bestmoderate stabilities. In particular, their voltage holding ratio dropsafter exposure. In addition, a yellowish discoloration often arises.

The use of various stabilizers in liquid-crystalline media is described,for example, in JP (S)55-023169 (A), JP (H)05-117324 (A), WO 02/18515 A1and JP (H) 09-291282 (A).

TINUVIN®123, a compound of the formula

has also been proposed for stabilization purposes.

Mesogenic compounds containing one or two HALS units are disclosed in EP1 1784 442 A1.

HALS with various substituents on the nitrogen atom are compared withrespect to their pK_(B) values in Ohkatsu, Y., J. of Japan PetroleumInstitute, 51, 2008, pages 191-204. The following types of structuralformulae are disclosed here.

Type Active group of the stabilizer “HALS”

“R-HALS” or “NR-HALS”

“NOR-HALS”

The compound TEMPOL, of the following formula:

is known; it is mentioned, for example, in Miéville, P. et al., Angew.Chem. 2010, 122, pages 6318-6321. It is commercially available fromvarious manufacturers and is employed, for example, as polymerisationinhibitor and, in particular in combination with UV absorbers, as lightor UV protection in formulations for precursors of polyolefins,polystyrenes, polyamides, coatings and PVC.

The liquid-crystal media of the prior art having correspondingly lowaddressing voltages have relatively low electrical resistance values ora low VHR and often result in undesired flicker and/or inadequatetransmission in the displays. In addition, they are not sufficientlystable to heating and/or UV exposure, at least if they havecorrespondingly high polarity, as is necessary for low addressingvoltages.

On the other hand, the addressing voltage of the displays of the priorart which have a high VHR is often too high, in particular for displayswhich are not connected directly or not continuously to the power supplynetwork, such as, for example, displays for mobile applications.

In addition, the phase range of the liquid-crystal mixture must besufficiently broad for the intended application of the display.

The response times of the liquid-crystal media in the displays must beimproved, i.e. reduced. This is particularly important for displays fortelevision or multimedia applications. In order to improve the responsetimes, it has repeatedly been proposed in the past to optimise therotational viscosity of the liquid-crystal media (γ₁), i.e. to achievemedia having the lowest possible rotational viscosity. However, theresults achieved here are inadequate for many applications and thereforemake it appear desirable to find further optimisation approaches.

Adequate stability of the media to extreme loads, in particular to UVexposure and heating, is very particularly important. In particular inthe case of applications in displays in mobile equipment, such as, forexample, mobile telephones, this may be crucial.

The disadvantage of the MLC displays disclosed hitherto is due to theircomparatively low contrast, the relatively high viewing-angle dependenceand the difficulty in producing grey shades in these displays, as wellas their inadequate VHR and their inadequate lifetime.

There thus continues to be a great demand for MLC displays having veryhigh specific resistance at the same time as a large working-temperaturerange, short response times and a low threshold voltage, with the aid ofwhich various grey shades can be produced and which have, in particular,a good and stable VHR.

The invention has the object of providing MLC displays, not only formonitor and TV applications, but also for mobile telephones andnavigation systems, which are based on the ECB or IPS effect, do nothave the disadvantages indicated above, or only do so to a lesserextent, and at the same time have very high specific resistance values.In particular, it must be ensured for mobile telephones and navigationsystems that they also work at extremely high and extremely lowtemperatures.

Surprisingly, it has been found that it is possible to achieveliquid-crystal displays which have, in particular in ECB displays, a lowthreshold voltage with short response times and at the same time asufficiently broad nematic phase, favorable, relatively lowbirefringence (Δn), good stability to decomposition by heating and by UVexposure, and a stable, high VHR if use is made in these displayelements of nematic liquid-crystal mixtures which comprise at least onecompound of the formula I and in each case at least one compound of theformula III-O, preferably of the sub-formulae III-O-1 and/or III-O-2,and preferably at least one compound of to formula II and/or at leastone compound selected from the group of the compounds of the formulaeIII-1 to III-4, preferably of the formula III-3.

Media of this type can be used, in particular, for electro-opticaldisplays having active-matrix addressing based on the ECB effect and forIPS (in-plane switching) displays.

The invention thus relates to a liquid-crystalline medium based on amixture of polar compounds which comprises at least one compound of theformula I and one or more compounds of the formula III-O and preferablyadditionally one or more compounds of the formula II and/or additionallyone or more compounds selected from the group of the compounds of theformulae III-1 to III-4.

The mixtures according to the invention exhibit very broad nematic phaseranges with clearing points ≧70° C., very favorable values for thecapacitive threshold, relatively high values for the holding ratio andat the same time good low-temperature stabilities at −20° C. and −30°C., as well as very low rotational viscosities. The mixtures accordingto the invention are furthermore distinguished by a good ratio ofclearing point and rotational viscosity and by a high negativedielectric anisotropy.

Surprisingly, it has now been found that it is possible to achieveliquid-crystalline media having a suitably high Δ∈, a suitable phaserange and Δn which do not have the disadvantages of the prior-artmaterials, or at least only do so to a considerably reduced extent.

Surprisingly, it has been found here that the compounds of the formulaI, even when used alone without additional heat stabilizers, result inconsiderable, in many cases adequate, stabilization of liquid-crystalmixtures both to UV exposure and also to heating. This is the case, inparticular, in most cases in which the parameter R¹¹ in the compounds ofthe formula I used denotes O. The compounds of the formula I in whichR¹¹ denotes O. are therefore particularly preferred, and the use ofprecisely these compounds in the liquid-crystal mixtures is particularlypreferred.

However, adequate stabilization of liquid-crystal mixtures both againstUV exposure and against heating can also be achieved, in particular, ifone or more further compounds, preferably phenolic stabilizers, arepresent in the liquid-crystal mixture in addition to the compound of theformula I, or the compounds of the formula I. These further compoundsare suitable as heat stabilizers.

The invention thus relates to compounds of the formula I, and to aliquid-crystalline medium having a nematic phase and negative dielectricanisotropy which comprises

-   a) one or more compounds of the formula I, preferably in a    concentration in the range from 1 ppm to 1000 ppm, preferably in the    range from 1 ppm to 500 ppm, particularly preferably in the range    from 1 ppm to 250 ppm,

-   -   in which    -   n denotes an integer from 1 to 4, preferably 1, 2 or 3,        particularly preferably 1 or 2, and very particularly preferably        2,    -   m denotes (4-n),

-   -   -   denotes an organic radical having 4 bonding sites,            preferably an alkanetetrayl unit having 1 to 20 C atoms, in            which, in addition to the m groups R¹² present in the            molecule, but independently thereof, a further H atom may be            replaced by R¹² or a plurality of further H atoms may be            replaced by R¹², preferably a straight-chain alkanetetrayl            unit having one valence on each of the two terminal C atoms,            in which one —CH₂— group or a plurality of —CH₂— groups may            be replaced by —O— or —(C═O)— in such a way that two O atoms            are not bonded directly to one another, or denotes a            substituted or unsubstituted aromatic or heteroaromatic            hydrocarbon radical having 1 to 4 valences, in which, in            addition to the m groups R¹² present in the molecule, but            independently thereof, a further H atom may be replaced by            R¹² or a plurality of further H atoms may be replaced by            R¹²,

    -   Z¹¹ and Z¹², independently of one another, denote —O—, —(C═O)—,        —(N—R¹⁴)— or a single bond, but do not both simultaneously        denote —O—,

    -   r and s, independently of one another, denote 0 or 1,

    -   Y¹¹ to Y¹⁴ each, independently of one another, denote alkyl        having 1 to 4 C atoms, preferably methyl or ethyl, particularly        preferably all denote either methyl or ethyl and very        particularly preferably methyl, and alternatively, also        independently of one another, one or both of the pairs (Y¹¹ and        Y¹²) and (Y¹³ and Y¹⁴) together denote a divalent group having 3        to 6 C atoms, preferably having 5 C atoms, particularly        preferably 1,5-pentylene

    -   R¹¹ denotes O—R¹³, O. or OH, preferably O—R¹³ or O.,        particularly preferably O., OH, isopropoxy, cyclohexyloxy,        acetophenoxy or benzoxy and very particularly preferably OH,

R¹² on each occurrence, independently of one another, denotes H, F,OR¹⁴, NR¹⁴R¹⁶, a straight-chain or branched alkyl chain having 1-20 Catoms, in which one —CH₂— group or a plurality of —CH₂— groups may bereplaced by —O— or —C(═O)—, but two adjacent —CH₂— groups cannot bereplaced by —O—, or denotes a hydrocarbon radical which contains acycloalkyl or alkylcycloalkyl unit, and in which one —CH₂— group or aplurality of —CH₂— groups may be replaced by —O— or —C(═O)—, but twoadjacent —CH₂— groups cannot be replaced by —O—, and in which one H atomor a plurality of H atoms may be replaced by OR¹⁴, N(R¹⁴)(R¹⁵) or R¹⁶,or denotes an aromatic or heteroaromatic hydrocarbon radical, in whichone H atom or a plurality of H atoms may be replaced by OR¹⁴,N(R¹⁴)(R¹⁵) or R¹⁶,

-   -   R¹³ on each occurrence, independently of one another, denotes a        straight-chain or branched alkyl chain having 1-20 C atoms, in        which one —CH₂— group or a plurality of —CH₂— groups may be        replaced by —O— or —C(═O)—, but two adjacent —CH₂— groups cannot        be replaced by —O—, or denotes a hydrocarbon radical which        contains a cycloalkyl or alkylcycloalkyl unit, and in which one        —CH₂— group or a plurality of —CH₂— groups may be replaced by        —O— or —C(═O)—, but two adjacent —CH₂— groups cannot be replaced        by —O—, and in which one H atom or a plurality of H atoms may be        replaced by OR¹⁴, N(R¹⁴)(R¹⁵) or R¹⁶, or denotes an aromatic or        heteroaromatic hydrocarbon radical, in which one H atom or a        plurality of H atoms may be replaced by OR¹⁴, N(R¹⁴)(R¹⁵) or        R¹⁶, or can be

-   -   -   (1,4-cyclohexylene), in which one or more —CH₂— groups may            be replaced by —O—, —CO— or —NR¹⁴—, or an acetophenyl,            isopropyl or 3-heptyl radical,

    -   R¹⁴ on each occurrence, independently of one another, denotes a        straight-chain or branched alkyl or acyl group having 1 to 10 C        atoms, preferably n-alkyl, or an aromatic hydrocarbon or        carboxyl radical having 6-12 C atoms, preferably with the        proviso that, in the case of N(R¹⁴)(R¹⁵), at least one acyl        radical is present,

    -   R¹⁵ on each occurrence, independently of one another, denotes a        straight-chain or branched alkyl or acyl group having 1 to 10 C        atoms, preferably n-alkyl, or an aromatic hydrocarbon or        carboxyl radical having 6-12 C atoms, preferably with the        proviso that, in the case of N(R¹⁴)(R¹⁵), at least one acyl        radical is present,

    -   R¹⁶ on each occurrence, independently of one another, denotes a        straight-chain or branched alkyl group having 1 to 10 C atoms,        in which one —CH₂— group or a plurality of —CH₂— groups may be        replaced by —O— or —C(═O)—, but two adjacent —CH₂— groups cannot        be replaced by —O—,

    -   with the provisos that,

    -   in the case where n=1, R¹¹=O. and

    -   —[Z¹¹—]_(r)—[Z¹²]_(s)—=—O—, —(CO)—O—, —O—(CO)—, —O—(CO)—O—,        —NR¹⁴— or —NR¹⁴—(CO)—,

-   -   -   does not denote        -   straight-chain or branched alkyl having 1 to 10 C atoms,            also cycloalkyl, cycloalkylalkyl or alkylcycloalkyl, where            one or more —CH₂— groups in all these groups may be replaced            by —O— in such a way that two O atoms in the molecule are            not bonded directly to one another,        -   and

    -   in the case where n=2 and R¹¹=O—R¹³,

    -   R¹³ does not denote n-C₁₋₉-alkyl,        and

    -   compounds of the formula

-   -   are excluded from formula I, and

-   b) one or more compounds of the formula III-O

-   -   in which    -   R³¹ denotes an unsubstituted alkyl radical having 1 to 7 C        atoms, preferably an n-alkyl radical, particularly preferably        having 2 to 5 C atoms,    -   R³² denotes an unsubstituted alkyl radical having 1 to 7 C        atoms, preferably having 2 to 5 C atoms, or an unsubstituted        alkoxy radical having 1 to 6 C atoms, preferably having 1, 2, 3        or 4 C atoms, and    -   I denotes 1 or 2,    -   and

-   c) optionally, preferably obligatorily, one or more compounds of the    formula II

-   -   in which    -   R²¹ denotes an unsubstituted alkyl radical having 1 to 7 C atoms        or an unsubstituted alkenyl radical having 2 to 7 C atoms,        preferably an n-alkyl radical, particularly preferably having 3,        4 or 5 C atoms, and    -   R²² denotes an unsubstituted alkenyl radical having 2 to 7 C        atoms, preferably having 2, 3 or 4 C atoms, more preferably a        vinyl radical or a 1-propenyl radical and in particular a vinyl        radical,    -   and

-   d) optionally, preferably obligatorily, one or more compounds    selected from the group of the formulae III-1 to III-4, preferably    of the formula III-3,

-   -   in which    -   R³¹ denotes an unsubstituted alkyl radical having 1 to 7 C        atoms, preferably an n-alkyl radical, particularly preferably        having 2 to 5 C atoms,    -   R³² denotes an unsubstituted alkyl radical having 1 to 7 C        atoms, preferably having 2 to 5 C atoms, or an unsubstituted        alkoxy radical having 1 to 6 C atoms, preferably having 2, 3 or        4 C atoms, and    -   m, n and o each, independently of one another, denote 0 or 1.

The following embodiments are preferred:

-   -   which denotes

-   -   -   (benzene-1,2,4,5-tetrayl) or

    -   —CH₂—(CH—)—[CH₂]_(q)—(CH—)—CH₂— or

    -   >CH—[CH₂]_(p)—CH<, (where p∈{0, 1, 2, 3, 4, 5 to 18} and q∈{0,        1, 2, 3 to 16}) or

-   -   which denotes    -   >CH—[CH₂]_(p)—CH₂— (where p∈{0, 1, 2, 3, 4, 5 to 18}) or

-   -   which denotes    -   —CH₂— [CH₂]_(p)—CH₂— (where p∈{0, 1, 2, 3, 4, 5 to 18}),        propane-1,2-diyl, butane-1,2-diyl, ethane-1,2-diyl,

-   -   -   (1,4-phenylene),

-   -   -   (1,2-phenylene or

-   -   -   (1,4-cyclohexylene).

In the present application, the elements all include their respectiveisotopes. In particular, one or more H in the compounds may be replacedby D, and this is also particularly preferred in some embodiments. Acorrespondingly high degree of deuteration of the correspondingcompounds enables, for example, detection and recognition of thecompounds. This is very helpful in some cases, in particular in the caseof the compounds of the formula I.

In the present application,

-   alkyl particularly preferably denotes straight-chain alkyl, in    particular CH₃—, C₂H₅—, n-C₃H₇—, n-C₄H₉— or n-C₅H₁₁—, and-   alkenyl particularly preferably denotes CH₂═CH—, E-CH₃—CH═CH—,    CH₂═CH—CH₂—CH₂—, E-CH₃—CH═CH—CH₂—CH₂— or E-(n-C₃H₇)—CH═CH—.

The liquid-crystalline media in accordance with the present applicationpreferably comprise in total 1 ppm to 1000 ppm, preferably 1 ppm to 500ppm, even more preferably 1 to 250 ppm, preferably to 200 ppm and veryparticularly preferably 1 ppm to 100 ppm, of compounds of the formula I.

The concentration of the compounds of the formula I in the mediaaccording to the invention is preferably 90 ppm or less, particularlypreferably 50 ppm or less. The concentration of the compounds of theformula I in the media according to the invention is very particularlypreferably 10 ppm or more to 80 ppm or less.

In a preferred embodiment of the present invention, in the compounds ofthe formula I,

-   -   which denotes

-   -   (benzene-1,2,4,5-tetrayl) or

-   -   which denotes

-   -   (benzene-1,3,5-triyl) or

-   -   denotes —(CH₂—)₂, —(CH₂—)₄, —(CH₂—)₆, —(CH₂—)₈,        propane-1,2-diyl, butane-1,2-diyl, ethane-1,2-diyl,

-   -   (1,4-phenylene),

-   -   (1,3-phenylene),

-   -   (1,2-phenylene) or

-   -   (trans-1,4-cyclohexylene) and/or    -   —[Z¹¹]_(r)—[Z¹²—]_(s) on each occurrence, independently of one        another, denotes —O—, —(C═O)—O— or —O—(C═O)—, —(N—R¹⁴)— or a        single bond, preferably —O— or —(C═O)—O— or —O—(C═O)—, and/or    -   R¹¹ denotes —O., OH or O—R¹³, preferably:        -   —O., —O—CH(—CH₃)₂, —O—CH(—CH₃)(—CH₂)₃—CH₃,            —O—CH(—C₂H₅)(—CH₂)₃—CH₃,

-   -   -   and/or

    -   R¹², if present, denotes alkyl or alkoxy, and/or

    -   R¹³ denotes isopropyl or 3-heptyl, acetophenyl or cyclohexyl.

In a preferred embodiment of the present invention, the group

in the compounds of the formula I on each occurrence, independently ofone another, denotes

-   -   preferably

In a particularly preferred embodiment of the present invention, allgroups

present in the compounds of the formula I have the same meaning.

These compounds are highly suitable as stabilizers in liquid-crystalmixtures. In particular, they stabilise the VHR of the mixtures againstUV exposure.

In a preferred embodiment of the present invention, the media accordingto the invention in each case comprise one or more compounds of theformula I selected from the group of the compounds of the formulae I-1to I-9, preferably selected from the group of the compounds of theformulae I-1 to I-4,

-   -   in which the parameters have the meanings indicated above under        formula I, and    -   t denotes an integer from 1 to 12,    -   R¹⁷ denotes a straight-chain or branched alkyl chain having 1-12        C atoms, in which one —CH₂— group or a plurality of —CH₂— groups        may be replaced by —O— or —C(═O)—, but two adjacent —CH₂— groups        cannot be replaced by —O—, or denotes an aromatic or        heteroaromatic hydrocarbon radical, in which one H atom or a        plurality of H atoms may be replaced by OR¹⁴, N(R¹⁴)(R¹⁵) or        R¹⁶.

In an even more preferred embodiment of the present invention, the mediaaccording to the invention in each case comprise one or more compoundsof the formula I selected from the group of the following compounds ofthe formulae I-1a-1 to I-8a-1:

In an even more preferred embodiment of the present invention, the mediaaccording to the invention in each case comprise one or more compoundsof the formula I selected from the group of the following compounds ofthe formulae I-2a-1 and I-2a-2:

In an alternative, preferred embodiment of the present invention, themedia according to the invention in each case comprise one or morecompounds of the formula I selected from the group of the followingcompounds of the formulae I-1b-1 and I-1b-2:

In an alternative, preferred embodiment of the present invention, themedia according to the invention in each case comprise one or morecompounds of the formula I selected from the group of the followingcompounds of the formulae I-1c-1 and I-1c-2:

In a further alternative, preferred embodiment of the present invention,the media according to the invention in each case comprise one or morecompounds of the formula I selected from the group of the followingcompounds of the formulae I-1d-1 to I-1d-4:

In a further alternative, preferred embodiment of the present invention,the media according to the invention in each case comprise one or morecompounds of the formula I selected from the group of the followingcompounds of the formulae I-3d-1 to I-3d-8:

In a further alternative, preferred embodiment of the present invention,the media according to the invention in each case comprise one or morecompounds of the formula I selected from the group of the followingcompounds of the formulae I-4-d-1 and I-4d-2:

In a further alternative, preferred embodiment of the present invention,the media according to the invention in each case comprise one or morecompounds of the formula I selected from the group of the followingcompounds of the formulae I-1e-1 and I-1e-2:

In a further alternative, preferred embodiment of the present invention,the media according to the invention in each case comprise one or morecompounds of the formula I selected from the group of the followingcompounds of the formulae I-5e-1 to I-8e-1:

In addition to the compounds of the formula I or preferred sub-formulaethereof, the media in accordance with the present invention preferablycomprise one or more dielectrically neutral compounds of the formula IIin a total concentration in the range from 5% or more to 90% or less,preferably from 10% or more to 80% or less, particularly preferably from20% or more to 70% or less.

The medium according to the invention preferably comprises one or morecompounds selected from the group of the formulae III-1 to III-4 in atotal concentration in the range from 10% or more to 80% or less,preferably from 15% or more to 70% or less, particularly preferably from20% or more to 60% or less.

The medium according to the invention particularly preferably comprises

-   -   one or more compounds of the formula III-O in a total        concentration in the range from 5% or more to 40% or less,        preferably in the range from 10% or more to 30% or less, and/or    -   one or more compounds of the formula III-1 in a total        concentration in the range from 5% or more to 30% or less,        and/or    -   one or more compounds of the formula III-2 in a total        concentration in the range from 3% or more to 30% or less,        and/or    -   one or more compounds of the formula III-3 in a total        concentration in the range from 5% or more to 30% or less,        and/or    -   one or more compounds of the formula III-4 in a total        concentration in the range from 1% or more to 30% or less.

Preferred compounds of the formula II are the compounds selected fromthe group of the compounds of the formulae II-1 and II-2, preferably ofthe formula II-1,

in which

-   alkyl denotes an alkyl radical having 1 to 7 C atoms, preferably    having 2 to 5 C atoms,-   alkenyl denotes an alkenyl radical having 2 to 5 C atoms, preferably    having 2 to 4 C atoms, particularly preferably 2 C atoms,-   alkenyl′ denotes an alkenyl radical having 2 to 5 C atoms,    preferably having 2 to 4 C atoms, particularly preferably having 2    to 3 C atoms.

The media according to the invention preferably comprise one or morecompounds of the formula III-O selected from the group of the compoundsof the formulae III-O-1 and III-O-2, preferably one or more of each ofthe compounds of the formulae III-O-1 and III-O-2,

in which the parameters have the meaning given above in the case offormula III-O, and preferably

-   R³¹ denotes an alkyl radical having 2 to 5 C atoms, preferably    having 3 to 5 C atoms, and-   R³² denotes an alkyl or alkoxy radical having 2 to 5 C atoms,    preferably an alkoxy radical having 1 C atom or having 2 to 4 C    atoms, or an alkenyloxy radical having 2 to 4 C atoms.

In a preferred embodiment of the present invention, the media accordingto the invention comprise one or more compounds of the formula III-1,preferably one or more compounds selected from the group of thecompounds of the formulae III-1-1 and III-1-2,

in which the parameters have the meanings given above for formula III-1,and preferably

-   R³¹ denotes an alkyl radical having 2 to 5 C atoms, preferably    having 3 to 5 C atoms, and-   R³² denotes an alkyl or alkoxy radical having 2 to 5 C atoms,    preferably an alkoxy radical having 2 to 4 C atoms, or an alkenyloxy    radical having 2 to 4 C atoms.

The media according to the invention preferably comprise one or morecompounds of the formula III-2, preferably one or more compoundsselected from the group of the compounds of the formulae III-2-1 andIII-2-2,

in which the parameters have the meanings given above for formula III-2,and preferably

-   R³¹ denotes an alkyl radical having 2 to 5 C atoms, preferably    having 3 to 5 C atoms, and-   R³² denotes an alkyl or alkoxy radical having 2 to 5 C atoms,    preferably an alkoxy radical having 2 to 4 C atoms, or an alkenyloxy    radical having 2 to 4 C atoms.

In a particularly preferred embodiment of the present invention, themedia according to the invention comprise one or more compounds of theformula III-3, preferably one or more compounds selected from the groupof the compounds of the formulae III-3-1 and III-3-2, very particularlypreferably of the formula III-3-2,

in which the parameters have the meanings given above for formula III-3,and preferably

-   R³¹ denotes an alkyl radical having 2 to 5 C atoms, preferably    having 3 to 5 C atoms, and-   R³² denotes an alkyl or alkoxy radical having 2 to 5 C atoms,    preferably an alkoxy radical having 2 to 4 C atoms, or an alkenyloxy    radical having 2 to 4 C atoms.

In a preferred embodiment, the media according to the invention compriseone or more compounds of the formula II selected from the group of thecompounds of the formulae I′-1 and II-2.

In a different preferred embodiment, the media according to theinvention comprise no compounds of the formula II.

In a further preferred embodiment, the medium comprises one or morecompounds of the formula IV

in which

-   R⁴¹ denotes alkyl having 1 to 7 C atoms, preferably having 2 to 5 C    atoms, and-   R⁴² denotes alkyl having 1 to 7 C atoms or alkoxy having 1 to 6 C    atoms, both preferably having 2 to 5 C atoms.

In a further preferred embodiment, the medium comprises one or morecompounds of the formula V

in which

-   R⁵¹ and R⁵², independently of one another, have one of the meanings    given for R²¹ and R²² and preferably denote alkyl having 1 to 7 C    atoms, preferably n-alkyl, particularly preferably n-alkyl having 1    to 5 C atoms, alkoxy having 1 to 7 C atoms, preferably n-alkoxy,    particularly preferably n-alkoxy having 2 to 5 C atoms, alkoxyalkyl,    alkenyl or alkenyloxy having 2 to 7 C atoms, preferably having 2 to    4 C atoms, preferably alkenyloxy,

to

if present, each, independently of one another, denote

preferably

preferably

denotes

and, if present,

preferably denotes

-   Z⁵¹ to Z⁵³ each, independently of one another, denote —CH₂—CH₂—,    —CH₂—O—, —CH═CH—, —C≡C—, —COO— or a single bond, preferably    —CH₂—CH₂—, —CH₂—O— or a single bond and particularly preferably a    single bond,-   p and q each, independently of one another, denote 0 or 1,-   (p+q) preferably denotes 0 or 1.

The media according to the invention preferably comprise the followingcompounds in the total concentrations indicated:

-   -   10-60% by weight of one or more compounds selected from the        group of the compounds of the formulae III-O and III-1 to III-4        and/or    -   30-80% by weight of one or more compounds of the formulae IV        and/or V,    -   where the total content of all compounds in the medium is 100%.

In a particularly preferred embodiment, the media according to theinvention comprise one or more compounds selected from the group of thecompounds of the formulae OH-1 to OH-6,

These compounds are highly suitable for the heat stabilization of themedia.

In another preferred embodiment of the present invention, in which themedia according to the invention comprise, in particular, one or morecompounds of the formula I in which R¹¹, or at least one of R¹¹, denotesO., these media can also have adequate stability if they do not comprisea phenol compound, in particular selected from the group of thecompounds of the formulae OH-1 to OH-6.

In a further preferred embodiment of the present invention, the mediaaccording to the invention at least in each case comprise one or morecompounds of the formula I in which the groups R¹¹ of the one compoundof the formula I have a different meaning than in the other compounds ofthe formula I.

The present invention also relates to electro-optical displays orelectro-optical components which contain liquid-crystalline mediaaccording to the invention. Preference is given to electro-opticaldisplays which are based on the VA or ECB effect and in particular thosewhich are addressed by means of an active-matrix addressing device.

Accordingly, the present invention likewise relates to the use of aliquid-crystalline medium according to the invention in anelectro-optical display or in an electro-optical component, and to aprocess for the preparation of the liquid-crystalline media according tothe invention, characterised in that one or more compounds of theformula I are mixed with one or more compounds of the formula II,preferably with one or more compounds of the sub-formula II-1, and oneor more further compounds, preferably selected from the group of thecompounds of the formulae III-1 to III-4 and IV and/or V.

In addition, the present invention relates to a method for thestabilization of a liquid-crystalline medium which comprises one or morecompounds of the formula III-O, optionally one or more compounds of theformula II and/or one or more compounds selected from the group of thecompounds of the formulae III-1 to III-4, characterised in that one ormore compounds of the formula I are added to the medium.

In a further preferred embodiment, the medium comprises one or morecompounds of the formula IV, selected from the group of the compounds ofthe formulae IV-1 and IV-2,

in which

-   alkyl and alkyl′, independently of one another, denote alkyl having    1 to 7 C atoms, preferably having 2 to 5 C atoms,-   alkoxy denotes alkoxy having 1 to 5 C atoms, preferably having 2 to    4 C atoms.

In a further preferred embodiment, the medium comprises one or morecompounds of the formula V selected from the group of the compounds ofthe formulae V-1 to V-7 and VI-1 to VI-3, preferably selected from thegroup of the compounds of the formulae V-1 to V-5,

in which the parameters have the meanings given above under formula V,and

-   Y⁵ denotes H or F, and preferably-   R⁵¹ denotes alkyl having 1 to 7 C atoms or alkenyl having 2 to 7 C    atoms, and-   R⁵² denotes alkyl having 1 to 7 C atoms, alkenyl having 2 to 7 C    atoms or alkoxy having 1 to 6 C atoms, preferably alkyl or alkenyl,    particularly preferably alkenyl.

In a further preferred embodiment, the medium comprises one or morecompounds of the formula V-1 selected from the group of the compounds ofthe formulae V-1a and V-1b, preferably of the formula V-1b,

in which

-   alkyl and alkyl′, independently of one another, denote alkyl having    1 to 7 C atoms, preferably having 2 to 5 C atoms,-   alkoxy denotes alkoxy having 1 to 5 C atoms, preferably having 2 to    4 C atoms.

In a further preferred embodiment, the medium comprises one or morecompounds of the formula V-3 selected from the group of the compounds ofthe formulae V-3a and V-3b,

in which

-   alkyl and alkyl′, independently of one another, denote alkyl having    1 to 7 C atoms, preferably having 2 to 5 C atoms, and-   alkenyl denotes alkenyl having 2 to 7 C atoms, preferably having 2    to 5 C atoms.

In a further preferred embodiment, the medium comprises one or morecompounds of the formula V-4 selected from the group of the compounds ofthe formulae V-4a and V-4-b,

in which

-   alkyl and alkyl′, independently of one another, denote alkyl having    1 to 7 C atoms, preferably having 2 to 5 C atoms.

In a further preferred embodiment, the medium comprises one or morecompounds of the formula III-4, preferably of the formula III-4-a,

in which

-   alkyl and alkyl′, independently of one another, denote alkyl having    1 to 7 C atoms, preferably having 2 to 5 C atoms.

The liquid-crystal media in accordance with the present invention maycomprise one or more chiral compounds.

Particularly preferred embodiments of the present invention meet one ormore of the following conditions,

where the acronyms (abbreviations) are explained in Tables A to C andillustrated by examples in Table D.

-   i. The liquid-crystalline medium has a birefringence of 0.060 or    more, particularly preferably 0.070 or more.-   ii. The liquid-crystalline medium has a birefringence of 0.130 or    less, particularly preferably 0.120 or less.-   iii. The liquid-crystalline medium has a birefringence in the range    from 0.090 or more to 0.120 or less.-   iv. The liquid-crystalline medium has a negative dielectric    anisotropy having a value of 2.0 or more, particularly preferably    3.0 or more.-   v. The liquid-crystalline medium has a negative dielectric    anisotropy having a value of 5.5 or less, particularly preferably    4.0 or less.-   vi. The liquid-crystalline medium has a negative dielectric    anisotropy having a value in the range from 2.5 or more to 3.8 or    less.-   vii. The liquid-crystalline medium comprises one or more    particularly preferred compounds of the formula II selected from the    sub-formulae given below:

-    in which alkyl has the meaning given above and preferably, in each    case independently of one another, denotes alkyl having 1 to 6,    preferably having 2 to 5, C atoms and particularly preferably    n-alkyl.-   viii. The total concentration of the compounds of the formula II in    the mixture as a whole is 25% or more, preferably 30% or more, and    is preferably in the range from 25% or more to 49% or less,    particularly preferably in the range from 29% or more to 47% or    less, and very particularly preferably in the range from 37% or more    to 44% or less.-   ix. The liquid-crystalline medium comprises one or more compounds of    the formula II selected from the group of the compounds of the    following formulae: CC-n-V and/or CC-n-Vm, particularly preferably    CC-3-V, preferably in a concentration of up to 50% or less,    particularly preferably up to 42% or less, and optionally    additionally CC-3-V1, preferably in a concentration of up to 15% or    less, and/or CC-4-V, preferably in a concentration of up to 20% or    less, particularly preferably up to 10% or less.-   x. The total concentration of the compounds of the formula CC-3-V in    the mixture as a whole is 20% or more, preferably 25% or more.-   xi. The proportion of compounds of the formulae III-1 to III-4 in    the mixture as a whole is 50% or more and preferably 75% or less.-   xii. The liquid-crystalline medium essentially consists of compounds    of the formulae I, II, III-1 to III-4, IV and V, preferably of    compounds of the formulae I, II and III-1 to III-4.-   xiii. The liquid-crystalline medium comprises one or more compounds    of the formula IV, preferably in a total concentration of 5% or    more, in particular 10% or more, and very particularly preferably    15% or more to 40% or less.

The invention furthermore relates to an electro-optical display havingactive-matrix addressing based on the VA or ECB effect, characterised inthat it contains, as dielectric, a liquid-crystalline medium inaccordance with the present invention.

The liquid-crystal mixture preferably has a nematic phase range having awidth of at least 80 degrees and a flow viscosity ν₂₀ of at most 30mm²·s⁻¹ at 20° C.

The liquid-crystal mixture according to the invention has a Δ∈ of −0.5to −8.0, in particular −1.5 to −6.0, and very particularly preferably−2.0 to −5.0, where Δ∈ denotes the dielectric anisotropy.

The rotational viscosity γ₁ is preferably 120 mPa·s or less, inparticular 100 mPa·s or less.

The mixtures according to the invention are suitable for all VA-TFTapplications, such as, for example, VAN, MVA, (S)-PVA and ASV. They arefurthermore suitable for IPS (in-plane switching), FFS (fringe-fieldswitching) and PALC applications having negative Δ∈.

The nematic liquid-crystal mixtures in the displays according to theinvention generally comprise two components A and B, which themselvesconsist of one or more individual compounds.

The liquid-crystalline media according to the invention preferablycomprise 4 to 15, in particular 5 to 12, and particularly preferably 10or less, compounds. These are preferably selected from the group of thecompounds of the formulae I, II and III-1 to III-4, and/or IV and/or V.

The liquid-crystalline media according to the invention may optionallyalso comprise more than 18 compounds. In this case, they preferablycomprise 18 to 25 compounds.

Besides compounds of the formulae I to V, other constituents may also bepresent, for example in an amount of up to 45%, but preferably up to35%, in particular up to 10%, of the mixture as a whole.

The media according to the invention may optionally also comprise adielectrically positive component, whose total concentration ispreferably 10% or less, based on the entire medium.

In a preferred embodiment, the liquid-crystal media according to theinvention comprise in total, based on the mixture as a whole,

10 ppm or more to 1000 ppm or less, preferably 50 ppm or more to 500 ppmor less, particularly preferably 100 ppm or more to 400 ppm or less andvery particularly preferably 150 ppm or more to 300 ppm or less, of thecompound of the formula I,

20% or more to 60% or less, preferably 25% or more to 50% or less,particularly preferably 30% or more to 45% or less, of compounds of theformula II, and

50% or more to 70% or less of compounds of the formulae III-1 to III-4.

In a preferred embodiment, the liquid-crystal media according to theinvention comprise compounds selected from the group of the compounds ofthe formulae I, II, III-1 to III-4, IV and V, preferably selected fromthe group of the compounds of the formulae I, II and III-1 to III-4;they preferably consist predominantly, particularly preferablyessentially and very particularly preferably virtually completely of thecompounds of the said formulae.

The liquid-crystal media according to the invention preferably have anematic phase from in each case at least −20° C. or less to 70° C. ormore, particularly preferably from −30° C. or less to 80° C. or more,very particularly preferably from −40° C. or less to 85° C. or more andmost preferably from −40° C. or less to 90° C. or more.

The expression “have a nematic phase” here means on the one hand that nosmectic phase and no crystallisation are observed at low temperatures atthe corresponding temperature and on the other hand that no clearingoccurs on heating out of the nematic phase. The investigation at lowtemperatures is carried out in a flow viscometer at the correspondingtemperature and checked by storage in test cells having a cell thicknesscorresponding to the electro-optical application for at least 100 hours.If the storage stability at a temperature of −20° C. in a correspondingtest cell is 1000 h or more, the medium is regarded as stable at thistemperature. At temperatures of −30° C. and −40° C., the correspondingtimes are 500 h and 250 h respectively. At high temperatures, theclearing point is measured in capillaries by conventional methods.

In a preferred embodiment, the liquid-crystal media according to theinvention are characterised by optical anisotropy values in the moderateto low range. The birefringence values are preferably in the range from0.065 or more to 0.130 or less, particularly preferably in the rangefrom 0.080 or more to 0.120 or less and very particularly preferably inthe range from 0.085 or more to 0.110 or less.

In this embodiment, the liquid-crystal media according to the inventionhave negative dielectric anisotropy and relatively high absolute valuesof the dielectric anisotropy (|Δ∈|) which are preferably in the rangefrom 2.7 or more to 5.3 or less, preferably to 4.5 or less, preferablyfrom 2.9 or more to 4.5 or less, particularly preferably from 3.0 ormore to 4.0 or less and very particularly preferably from 3.5 or more to3.9 or less.

The liquid-crystal media according to the invention have relatively lowvalues for the threshold voltage (V₀) in the range from 1.7 V or more to2.5 V or less, preferably from 1.8 V or more to 2.4 V or less,particularly preferably from 1.9 V or more to 2.3 V or less and veryparticularly preferably from 1.95 V or more to 2.1 V or less.

In a further preferred embodiment, the liquid-crystal media according tothe invention preferably have relatively low values of the averagedielectric anisotropy (∈_(av.)≡(∈_(∥)+2∈_(⊥))/3) which are preferably inthe range from 5.0 or more to 7.0 or less, preferably from 5.5 or moreto 6.5 or less, still more preferably from 5.7 or more to 6.4 or less,particularly preferably from 5.8 or more to 6.2 or less and veryparticularly preferably from 5.9 or more to 6.1 or less.

In addition, the liquid-crystal media according to the invention havehigh values for the VHR in liquid-crystal cells.

In freshly filled cells at 20° C. in the cells, these are greater thanor equal to 95%, preferably greater than or equal to 97%, particularlypreferably greater than or equal to 98% and very particularly preferablygreater than or equal to 99%, and after 5 minutes in the oven at 100° C.in the cells, these are greater than or equal to 90%, preferably greaterthan or equal to 93%, particularly preferably greater than or equal to96% and very particularly preferably greater than or equal to 98%.

In general, liquid-crystal media having a low addressing voltage orthreshold voltage here have a lower VHR than those having a higheraddressing voltage or threshold voltage, and vice versa.

These preferred values for the individual physical properties arepreferably also in each case maintained by the media according to theinvention in combination with one another.

In the present application, the term “compounds”, also written as“compound(s)”, means both one and also a plurality of compounds, unlessexplicitly indicated otherwise.

Unless indicated otherwise, the individual compounds are generallyemployed in the mixtures in concentrations in each case from 1% or moreto 30% or less, preferably from 2% or more to 30% or less andparticularly preferably from 3% or more to 16% or less.

In a preferred embodiment, the liquid-crystalline media according to theinvention comprise

the compound of the formula I,

one or more compounds of the formula II, preferably selected from thegroup of the compounds of the formulae CC-n-V and CC-n-Vm, preferablyCC-3-V, CC-3-V1, CC-4-V and CC-5-V, particularly preferably selectedfrom the group of the compounds CC-3-V, CC-3-V1 and CC-4-V, veryparticularly preferably the compound CC-3-V, and optionally additionallythe compound(s) CC-4-V and/or CC-3-V1,one or more compounds of the formula III-1-1, preferably of the formulaCY-n-Om, selected from the group of the compounds of the formulaeCY-3-O2, CY-3-O4, CY-5-O2 and CY-5-O4, one or more compounds of theformula III-1-2, preferably selected from the group of the compounds ofthe formulae CCY-n-m and CCY-n-Om, preferably of the formula CCY-n-Om,preferably selected from the group of the compounds of the formulaeCCY-3-O2, CCY-2-O2, CCY-3-O1, CCY-3-O3, CCY-4-O2, CCY-3-O2 and CCY-5-O2,optionally, preferably obligatorily, one or more compounds of theformula III-2-2, preferably of the formula CLY-n-Om, preferably selectedfrom the group of the compounds of the formulae CLY-2-O4, CLY-3-O2,CLY-3-O3,one or more compounds of the formula III-3-2, preferably of the formulaCPY-n-Om, preferably selected from the group of the compounds of theformulae CPY-2-O2 and CPY-3-O2, CPY-4-O2 and CPY-5-O2,one or more compounds of the formula III-4, preferably of the formulaPYP-n-m, preferably selected from the group of the compounds of theformulae PYP-2-3 and PYP-2-4.

For the present invention, the following definitions apply in connectionwith the specification of the constituents of the compositions, unlessindicated otherwise in individual cases:

-   -   “comprise”: the concentration of the constituents in question in        the composition is preferably 5% or more, particularly        preferably 10% or more, very particularly preferably 20% or        more,    -   “predominantly consist of”: the concentration of the        constituents in question in the composition is preferably 50% or        more, particularly preferably 55% or more and very particularly        preferably 60% or more,    -   “essentially consist of”: the concentration of the constituents        in question in the composition is preferably 80% or more,        particularly preferably 90% or more and very particularly        preferably 95% or more, and    -   “virtually completely consist of”: the concentration of the        constituents in question in the composition is preferably 98% or        more, particularly preferably 99% or more and very particularly        preferably 100.0%.

This applies both to the media as compositions with their constituents,which can be components and compounds, and also to the components withtheir constituents, the compounds. Only in relation to the concentrationof an individual compound relative to the medium as a whole does theterm comprise mean: the concentration of the compound in question ispreferably 1% or more, particularly preferably 2% or more, veryparticularly preferably 4% or more.

For the present invention, “≦” means less than or equal to, preferablyless than, and “≧” means greater than or equal to, preferably greaterthan.

For the present invention,

denote trans-1,4-cyclohexylene, and

denote 1,4-phenylene.

For the present invention, the expression “dielectrically positivecompounds” means compounds having a Δ∈ of >1.5, the expression“dielectrically neutral compounds” means those where −1.5≦Δ∈≦1.5 and theexpression “dielectrically negative compounds” means those whereΔ∈<−1.5. The dielectric anisotropy of the compounds is determined hereby dissolving 10% of the compounds in a liquid-crystalline host anddetermining the capacitance of the resultant mixture in each case in atleast one test cell having a cell thickness of 20 μm with homeotropicand with homogeneous surface alignment at 1 kHz. The measurement voltageis typically 0.5 V to 1.0 V, but is always lower than the capacitivethreshold of the respective liquid-crystal mixture investigated.

The host mixture used for dielectrically positive and dielectricallyneutral compounds is ZLI-4792 and that used for dielectrically negativecompounds is ZLI-2857, both from Merck KGaA, Germany. The values for therespective compounds to be investigated are obtained from the change inthe dielectric constant of the host mixture after addition of thecompound to be investigated and extrapolation to 100% of the compoundemployed. The compound to be investigated is dissolved in the hostmixture in an amount of 10%. If the solubility of the substance is toolow for this purpose, the concentration is halved in steps until theinvestigation can be carried out at the desired temperature.

The liquid-crystal media according to the invention may, if necessary,also comprise further additives, such as, for example, stabilizersand/or pleo-chroic dyes and/or chiral dopants in the usual amounts. Theamount of these additives employed is preferably in total 0% or more to10% or less, based on the amount of the entire mixture, particularlypreferably 0.1% or more to 6% or less. The concentration of theindividual compounds employed is preferably 0.1% or more to 3% or less.The concentration of these and similar additives is generally not takeninto account when specifying the concentrations and concentration rangesof the liquid-crystal compounds in the liquid-crystal media.

In a preferred embodiment, the liquid-crystal media according to theinvention comprise a polymer precursor which comprises one or morereactive compounds, preferably reactive mesogens, and, if necessary,also further additives, such as, for example, polymerisation initiatorsand/or polymerisation moderators, in the usual amounts. The amount ofthese additives employed is in total 0% or more to 10% or less, based onthe amount of the entire mixture, preferably 0.1% or more to 2% or less.The concentration of these and similar additives is not taken intoaccount when specifying the concentrations and concentration ranges ofthe liquid-crystal compounds in the liquid-crystal media.

The compositions consist of a plurality of compounds, preferably 3 ormore to 30 or fewer, particularly preferably 6 or more to 20 or fewerand very particularly preferably 10 or more to 16 or fewer compounds,which are mixed in a conventional manner. In general, the desired amountof the components used in lesser amount is dissolved in the componentsmaking up the principal constituent of the mixture. This isadvantageously carried out at elevated temperature. If the selectedtemperature is above the clearing point of the principal constituent,completion of the dissolution operation is particularly easy to observe.However, it is also possible to prepare the liquid-crystal mixtures inother conventional ways, for example using pre-mixes or from a so-called“multibottle system”.

The mixtures according to the invention exhibit very broad nematic phaseranges having clearing points of 65° C. or more, very favorable valuesfor the capacitive threshold, relatively high values for the holdingratio and at the same time very good low-temperature stabilities at −30°C. and −40° C. Furthermore, the mixtures according to the invention aredistinguished by low rotational viscosities γ₁.

It goes without saying to the person skilled in the art that the mediaaccording to the invention for use in VA, IPS, FFS or PALC displays mayalso comprise compounds in which, for example, H, N, O, Cl, F have beenreplaced by the corresponding isotopes.

The structure of the liquid-crystal displays according to the inventioncorresponds to the usual geometry, as described, for example, in EP-A 0240 379.

The liquid-crystal phases according to the invention can be modified bymeans of suitable additives in such a way that they can be employed inany type of, for example, ECB, VAN, IPS, GH or ASM-VA LCD display thathas been disclosed to date.

Table E below indicates possible dopants which can be added to themixtures according to the invention. If the mixtures comprise one ormore dopants, it is (they are) employed in amounts of 0.01 to 4%,preferably 0.1 to 1.0%.

Stabilizers which can be added, for example, to the mixtures accordingto the invention, preferably in amounts of 0.01 to 6%, in particular 0.1to 3%, are shown below in Table F.

For the purposes of the present invention, all concentrations are,unless explicitly noted otherwise, indicated in percent by weight andrelate to the corresponding mixture or mixture component, unlessexplicitly indicated otherwise.

All temperature values indicated in the present application, such as,for example, the melting point T(C,N), the smectic (S) to nematic (N)phase transition T(S,N) and the clearing point T(N,I), are indicated indegrees Celsius (° C.) and all temperature differences arecorrespondingly indicated in differential degrees (° or degrees), unlessexplicitly indicated otherwise.

For the present invention, the term “threshold voltage” relates to thecapacitive threshold (V₀), also known as the Freedericks threshold,unless explicitly indicated otherwise.

All physical properties are and have been determined in accordance with“Merck Liquid Crystals, Physical Properties of Liquid Crystals”, statusNovember 1997, Merck KGaA, Germany, and apply for a temperature of 20°C., and Δn is determined at 589 nm and Δ∈ at 1 kHz, unless explicitlyindicated otherwise in each case.

The electro-optical properties, for example the threshold voltage (V₀)(capacitive measurement), are, as is the switching behaviour, determinedin test cells produced at Merck Japan. The measurement cells havesoda-lime glass substrates and are constructed in an ECB or VAconfiguration with polyimide alignment layers (SE-1211 with diluent **26(mixing ratio 1:1), both from Nissan Chemicals, Japan), which have beenrubbed perpendicularly to one another and effect homeotropic alignmentof the liquid crystals. The surface area of the transparent, virtuallysquare ITO electrodes is 1 cm².

Unless indicated otherwise, a chiral dopant is not added to theliquid-crystal mixtures used, but the latter are also particularlysuitable for applications in which doping of this type is necessary.

The VHR is determined in test cells produced at Merck Japan. Themeasurement cells have soda-lime glass substrates and are constructedwith polyimide alignment layers (AL-3046 from Japan Synthetic Rubber,Japan) with a layer thickness of 50 nm, which have been rubbedperpendicularly to one another. The layer thickness is a uniform 6.0 μm.The surface area of the transparent ITO electrodes is 1 cm².

The VHR is determined at 20° C. (VHR₂₀) and after 5 minutes in an ovenat 100° C. (VHR₁₀₀) in a commercially available instrument from AutronicMelchers, Germany. The voltage used has a frequency of 60 Hz.

The accuracy of the VHR measurement values depends on the respectivevalue of the VHR. The accuracy decreases with decreasing values. Thedeviations generally observed in the case of values in the variousmagnitude ranges are compiled in their order of magnitude in thefollowing table.

Deviation VHR range (relative) VHR values Δ_(G)VHR/VHR/% from to Approx.99.6%   100%  +/−0.1 99.0%   99.6%   +/−0.2 98% 99% +/−0.3 95% 98%+/−0.5 90% 95% +/−1 80% 90% +/−2 60% 80% +/−4 40% 60% +/−8 20% 40% +/−1010% 20% +/−20

The stability to UV irradiation is investigated in a “Suntest CPS”, acommercial instrument from Heraeus, Germany. The sealed test cells areirradiated for 2.0 hours without additional heating. The irradiationpower in the wavelength range from 300 nm to 800 nm is 765 W/m² V. A UV“cut-off” filter having an edge wavelength of 310 nm is used in order tosimulate the so-called window glass mode. In each series of experiments,at least four test cells are investigated for each condition, and therespective results are indicated as averages of the correspondingindividual measurements.

The decrease in the voltage holding ratio (ΔVHR) usually caused by theexposure, for example by UV irradiation by LCD backlighting, isdetermined in accordance with the following equation (1):ΔVHR(t)=VHR(t)−VHR(t=0)  (1).

A further characteristic quantity which, besides the VHR, cancharacterise the conductivity of the liquid-crystal mixtures is the iondensity. High values of the ion density often result in the occurrenceof display faults, such as image sticking and flickering. The iondensity is preferably determined in test cells produced at Merck JapanLtd. The test cells have substrates made from soda-lime glass and aredesigned with polyimide alignment layers (AL-3046 from Japan SyntheticRubber, Japan) having a polyimide layer thickness of 40 nm. The layerthickness of the liquid-crystal mixture is a uniform 5.8 μm. The area ofthe circular, transparent ITO electrodes, which are additionally fittedwith a guard ring, is 1 cm². The accuracy of the measurement method isabout ±15%. The cells are dried overnight in an oven at 120° C. beforefilling with the relevant liquid-crystal mixture.

The ion density is measured using a commercially available instrumentfrom TOYO, Japan. The measurement method is essentially a measurementmethod which is analogous to cyclic voltammetry, as described in M.Inoue, “Recent Measurement of Liquid Crystal Material Characteristics”,Proceedings IDW 2006, LCT-7-1,647. In this method, an applied directvoltage is varied between a positive and negative maximum value inaccordance with a pre-specified triangular profile. A complete runthrough the profile thus forms one measurement cycle. If the appliedvoltage is sufficiently large that the ions in the field are able tomove to the respective electrode, an ion current forms due to dischargeof the ions. The amount of charge transferred here is typically in therange from a few pC to a few nC. This makes highly sensitive detectionnecessary, which is ensured by the above-mentioned instrument. Theresults are depicted in a current/voltage curve. The ion current here isevident from the occurrence of a peak at voltages which are smaller thanthe threshold voltage of the liquid-crystal mixture. Integration of thepeak area gives the value for the ion density of the mixtureinvestigated. Four test cells are measured per mixture. The repetitionfrequency of the triangular voltage is 0.033 Hz, the measurementtemperature is 60° C., the maximum voltage is ±3 V to ±10 V, dependingon the magnitude of the dielectric anisotropy of the relevant mixture.

The rotational viscosity is determined using the rotating permanentmagnet method and the flow viscosity in a modified Ubbelohde viscometer.For liquid-crystal mixtures ZLI-2293, ZLI-4792 and MLC-6608, allproducts from Merck KGaA, Darmstadt, Germany, the rotational viscosityvalues determined at 20° C. are 161 mPa·s, 133 mPa·s and 186 mPa·srespectively, and the flow viscosity values (ν) are 21 mm²·s⁻¹, 14mm²·s⁻¹ and 27 mm²·s⁻¹ respectively.

The following symbols are used, unless explicitly indicated otherwise:

-   V₀ threshold voltage, capacitive [V] at 20° C.,-   n_(e) extraordinary refractive index measured at 20° C. and 589 nm,-   n_(o) ordinary refractive index measured at 20° C. and 589 nm,-   Δn optical anisotropy measured at 20° C. and 589 nm,-   ∈_(⊥) dielectric susceptibility perpendicular to the director at    20° C. and 1 kHz,-   ∈_(∥) dielectric susceptibility parallel to the director at 20° C.    and 1 kHz,-   Δ∈ dielectric anisotropy at 20° C. and 1 kHz,-   cl.p. or T(N,I) clearing point [° C.],-   ν flow viscosity measured at 20° C. [mm²·s⁻¹],-   γ₁ rotational viscosity measured at 20° C. [mPa·s],-   K₁ elastic constant, “splay” deformation at 20° C. [pN],-   K₂ elastic constant, “twist” deformation at 20° C. [pN],-   K₃ elastic constant, “bend” deformation at 20° C. [pN], and-   LTS low-temperature stability of the phase, determined in test    cells,-   VHR voltage holding ratio,-   ΔVHR decrease in the voltage holding ratio,-   S_(rel) relative stability of the VHR.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the examples, all temperatures are set forthuncorrected in degrees Celsius and, all parts and percentages are byweight, unless otherwise indicated.

The entire disclosures of all applications, patents and publications,cited herein and of corresponding DE application No. 10 2012 020 374.8,filed Oct. 18, 2012, are incorporated by reference herein.

The following examples explain the present invention without limitingit. However, they show the person skilled in the art preferred mixtureconcepts with compounds preferably to be employed and the respectiveconcentrations thereof and combinations thereof with one another. Inaddition, the examples illustrate the properties and propertycombinations that are accessible.

For the present invention and in the following examples, the structuresof the liquid-crystal compounds are indicated by means of acronyms, withthe transformation into chemical formulae taking place in accordancewith Tables A to C below. All radicals C_(n)H_(2n+1), C_(m)H_(2m+1) andC_(l)H_(2m+1) or C_(n)H_(2n), C_(m)H_(2m) and C_(l)H_(2l) arestraight-chain alkyl radicals or alkylene radicals, in each case havingn, m and l C atoms respectively. Table A shows the codes for the ringelements of the nuclei of the compound, Table B lists the bridgingunits, and Table C lists the meanings of the symbols for the left- andright-hand end groups of the molecules. The acronyms are composed of thecodes for the ring elements with optional linking groups, followed by afirst hyphen and the codes for the left-hand end group, and a secondhyphen and the codes for the right-hand end group. Table D showsillustrative structures of compounds together with their respectiveabbreviations.

TABLE A Ring elements C

D

Dl

A

Al

P

G

Gl

U

Ul

Y

P(F, Cl)Y

P(Cl, F)Y

np

n3f

nN3fl

th

thl

tH2f

tH2fl

o2f

o2fl

dh

K

Kl

L

Ll

F

Fl

TABLE B Bridging units E —CH₂—CH₂— V —CH═CH— T —C≡C— W —CF₂—CF₂— B—CF═CF— Z —CO—O— Zl —O—CO— X —CF═CH— Xl —CH═CF— O —CH₂—O— Ol —O—CH₂— Q—CF₂—O— Ql —O—CF₂—

TABLE C End groups On the left individually or On the right individuallyor in combination in combination -n- C_(n)H_(2n+1)— -n —C_(n)H_(2n+1)-nO- C_(n)H_(2n+1)—O— -nO —O—C_(n)H_(2n+1) -V- CH₂═CH— -V —CH═CH₂ -nV-C_(n)H_(2n+1)—CH═CH— -nV —C_(n)H_(2n)—CH═CH₂ -Vn- CH₂═CH—C_(n)H_(2n)—-Vn —CH═CH—C_(n)H_(2n+1) -nVm- C_(n)H_(2n+1)—CH═CH—C_(m)H_(2m)— -nVm—C_(n)H_(2n)—CH═CH—C_(m)H_(2m+1) -N- N≡C— -N —C≡N -S- S═C═N— -S —N═C═S-F- F— -F —F -CL- Cl— -CL —Cl -M- CFH₂— -M —CFH₂ -D- CF₂H— -D —CF₂H -T-CF₃— -T —CF₃ -MO- CFH₂O— -OM —OCFH₂ -DO- CF₂HO— -OD —OCF₂H -TO- CF₃O—-OT —OCF₃ -A- H—C≡C— -A —C≡C—H -nA- C_(n)H_(2n+1)—C≡C— -An—C≡C—C_(n)H_(2n+1) -NA- N≡C—C≡C— -AN —C≡C—C≡N On the left only incombination On the right only in combination - . . . n . . . -—C_(n)H_(2n)— - . . . n . . . —C_(n)H_(2n)— - . . . M . . . - —CFH— - .. . M . . . —CFH— - . . . D . . . - —CF₂— - . . . D . . . —CF₂— - . . .V . . . - —CH═CH— - . . . V . . . —CH═CH— - . . . I . . . - —CO—O— - . .. I . . . —CO—O— - . . . ZI . . . - —O—CO— - . . . ZI . . . —O—CO— - . .. K . . . - —CO— - . . . K . . . —CO— - . . . W . . . - —CF═CF— - . . .W . . . —CF═CF— in which n and m are each integers, and the three dots “. . . ” are place-holders for other abbreviations from this table.

Besides the compounds of the formula I, the mixtures according to theinvention preferably comprise one or more compounds of the compoundsmentioned below.

The following abbreviations are used:

-   -   (m, m and z are, independently of one another, each an integer,        preferably 1 to 6)

TABLE D

Table E shows chiral dopants which are preferably employed in themixtures according to the invention.

TABLE E

In a preferred embodiment of the present invention, the media accordingto the invention comprise one or more compounds selected from the groupof the compounds from Table E.

Table F shows stabilizers which can preferably be employed in themixtures according to the invention in addition to the compounds of theformula I. The parameter n here denotes an integer in the range from 1to 12. In particular, the phenol derivatives shown can be employed asadditional stabilizers since they act as antioxidants.

TABLE F

In a preferred embodiment of the present invention, the media accordingto the invention comprise one or more compounds selected from the groupof the compounds from Table F, in particular one or more compoundsselected from the group of the compounds of the two formulae

EXAMPLES

The following examples explain the present invention without restrictingit in any way. However, the physical properties make it clear to theperson skilled in the art what properties can be achieved and in whatranges they can be modified. In particular, the combination of thevarious properties which can preferably be achieved is thus well definedfor the person skilled in the art.

Substance Examples

The following substances are preferred substances of the formula I inaccordance with the present application or substances of the formula Ipreferably to be employed in accordance with the present application.

Synthesis Example 1 Synthesis ofbis(2,2,6,6-tetramethyl-4-piperidyl)-N,N′-dioxyl succinate (SubstanceExample 1)

2.15 g (12.26 mmol) of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, 40mg (0.33 mmol) of 4-(dimethylamino)pyridine and 1 ml (12.4 mmol) ofdried pyridine are initially introduced in 20 ml of dry dichloromethane.4 angstrom activated molecular sieve is subsequently added, and themixture is stirred at room temperature (RT for short; about 22° C.) for90 min. The reaction solution is cooled to a temperature in the rangefrom 7 to 10° C., and 0.71 ml (6.13 mmol) of succinyl dichloride isslowly added, and the mixture is stirred at RT for 18 h. Sufficient sat.NaHCO₃ solution and dichloromethane are added to the reaction solution,and the organic phase is separated off, washed with water and sat. NaClsolution, dried over Na₂SO₄, filtered and evaporated. The crude productis purified over silica gel with dichloromethane/methyl tert-butyl ether(95:5), giving the product as a white solid having a purity of >99.5%.

Synthesis Example 2 Synthesis ofbis(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl)decanedioate (SubstanceExample 4)

28.5 g (166 mmol) of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (freeradical) and 250 mg (2.05 mmol) of 4-(dimethylamino)pyridine aredissolved in 300 ml of degassed dichloromethane, and 50.0 ml (361 mmol)of triethylamine are added. The mixture is subsequently degassed andcooled to 0° C., and 10 g (41.4 mmol) of sebacoyl chloride dissolved in100 ml of degassed dichloromethane are added dropwise at 0-5° C., andthe mixture is stirred at room temperature for 18 h. When the reactionis complete, water and HCl (pH=4-5) are added with ice-cooling, and themixture is stirred for a further 30 min. The organic phase is separatedoff, and the water phase is subsequently extracted with dichloromethane,and the combined phases are washed with saturated NaCl solution anddried over Na₂SO₄, filtered and evaporated, giving 24.4 g of a redliquid, which together are passed through 100 g of basic Al₂O₃ and 500 gof silica gel on a frit with dichloromethane/methyl tert-butyl ether(95/5), giving orange crystals, which are dissolved in degassedacetonitrile at 50° C. and crystallised at −25° C., giving the productas orange crystals having an HPLC purity of 99.9%.

Synthesis Example 3 Synthesis ofbis(2,2,6,6-tetramethyl-4-piperidyl)-N,N′-dioxylbutanediol (SubstanceExample 7)

Sufficient pentane is added to 15.0 g (60% in mineral oil, 375 mmol) ofNaH under a protective gas, and the mixture is allowed to settle. Thepentane supernatant is pipetted off and carefully quenched withisopropanol with cooling. 100 ml of THF are then carefully added to thewashed NaH. The reaction mixture is heated to 55° C., and a solution of50.0 g (284 mmol) of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl in400 ml of THF is carefully added dropwise. The hydrogen formed isdischarged directly. When the addition of the solution is complete,stirring is continued at 60° C. overnight (16 h). The reaction mixtureis subsequently cooled to 5° C., and 1,4-butanediol dimethylsulfonate isadded in portions. The mixture is subsequently slowly heated to 60° C.and stirred at this temperature for 16 h. When the reaction is complete,the mixture is cooled to RT, and 200 ml of 6% ammonia solution in waterare added with cooling, and the mixture is stirred for 1 h. The organicphase is subsequently separated off, the aqueous phase is rinsed withmethyl tert-butyl ether, the combined organic phases are washed withsat. NaCl solution, dried and evaporated. The crude product is purifiedover silica gel with dichloromethane/methyl tert-butyl ether (8:2) andcrystallised from acetonitrile at −20° C., giving the product as a pinkcrystalline solid having a purity of >99.5%.

Synthesis Example 4 Synthesis ofbis[2,2,6,6-tetramethyl-1-(1-phenyl-ethoxy)piperidin-4-yl]succinate(Substance Example 24)

Step 4.1: Synthesis of2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperidin-4-ol

5.0 g (29.03 mmol) of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl,7.80 g (58.1 mmol) of 2-phenylpropionaldehyde and 100.6 mg (1.02 mmol)of copper(I) chloride are initially introduced in 20 ml of tert-butanol.6.45 ml (58.06 mmol) of 35% hydrogen peroxide solution are thencarefully and slowly added dropwise at such a rate that the internaltemperature does not exceed 30° C. The mixture is therefore cooled bymeans of ice-cooling during the dropwise addition. Oxygen is formed inthe reaction and would spontaneously be released in large quantities ifthe addition were too fast and the temperature too high. When theaddition is complete, the reaction solution is stirred at RT for afurther 16 h, and sufficient water/methyl tert-butyl ether issubsequently added, and the organic phase is separated off. The organicphase is washed with 10% ascorbic acid until peroxide-free, and theperoxide content is checked. The mixture is subsequently washed with 10%NaOH solution, water and sat. NaCl solution, dried over Na₂SO₄, filteredand evaporated. The crude product obtained is purified over silica gelwith heptane/methyl tert-butyl ether (1:1), giving the product ascolorless crystals.

Step 4.2: Synthesis ofbis[2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperidin-4-yl]succinate

1.52 g (5.5 mmol) of the product from the preceding step, the compound2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperidin-4-ol, 15.3 mg (0.125mmol) of dimethylaminopyridine and 1.02 ml (12.6 mmol) of dried pyridineare initially introduced in 10 ml of dichloromethane and cooled to atemperature in the range from 7 to 10° C. 0.255 ml (2.199 mmol) ofsuccinoyl dichloride is then added dropwise as such and if necessarytopped up if hydroxyl compound is still present. When the reaction iscomplete, the reaction mixture is filtered directly through silica gelwith dichloromethane and subsequently eluted with heptane/methyltert-butyl ether (1:1) and pure methyl tert-butyl ether. The productobtained is dissolved in acetonitrile and purified by means ofpreparative HPLC (2 Chromolith columns with 50 ml/min of acetonitrile),giving the product as a yellow oil having a purity of >99.9%.

Synthesis Example 5 Synthesis of2,2,6,6-tetramethyl-1-(1-phenyl-ethoxy)piperidin-4-yl pentanoate(Substance Example 31)

2.5 g (9.01 mmol) of the compound2,2,6,6-tetramethyl-1-(1-phenyl-ethoxy)piperidin-4-ol from step 3.1 and55.1 mg (0.45 mmol) of (4-dimethylaminopyridine) are dissolved in 50.0ml of dry dichloromethane and cooled to 3° C. 5.47 ml (27.03 mmol) ofvaleric anhydride are added at this temperature, and the mixture isstirred at room temperature for 14 h. When the reaction is complete, themixture is carefully poured into ice-water, adjusted to pH 6 using 2NHCl, and the organic phase is separated off. The aqueous phase isextracted with dichloromethane, and the combined organic phases arewashed with saturated NaCl solution, a mixture of water andtriethylamine (300:50 ml) and dried over MgSO₄, filtered and evaporated.Purification on silica gel with heptane/methyl tert-butyl ether (9:1)gives the product as a colorless oil.

Synthesis Example 6 Synthesis of1,4-bis(1-hydroxy-2,2,6,6-tetramethyl-4-piperidinyl)butanedioate(Substance Example 49)

40 ml of water and 80 ml of dioxane are mixed and carefully degassed bymeans of a stream of argon. 2.0 g (4.7 mmol) of the free radical fromSubstance Example 1 (Synthesis Example 1) are dissolved in the solventmixture, and 4.95 g (28.1 mmol) of ascorbic acid are added in portions.The reaction mixture becomes colorless during this addition and isstirred at 40° C. for 18 h under a protective-gas atmosphere. Themixture is cooled to room temperature, and 100 ml of water are added,the mixture is stirred briefly, and the crystals formed are filtered offwith suction. The crystals are dissolved in 50 ml of hot degassed THF,and the insoluble constituents are filtered off, and the filtrate iscrystallised at −25° C. The pale-pink crystals are then washed bystirring in acetonitrile at room temperature for 18 h, giving theproduct as pale-pink crystals having an HPLC purity of −100%.

Synthesis Example 7 Synthesis of1,10-bis(1-hydroxy-2,2,6,6-tetramethyl-4-piperidinyl)decanedioate(Substance Example 50)

All solvents used are thoroughly degassed in advance by means of astream of argon. During work-up, brown glass equipment must be used.1.70 g (3.32 mmol) of the free radical from Substance Example 4(Synthesis Example 2) are dissolved in 60 ml of dioxane. 3.6 g (20 mmol)of ascorbic acid dissolved in 30 ml of water are subsequently addeddropwise to the solution at room temperature. The reaction solutionstarts to become colorless during this dropwise addition, and thereaction is complete after stirring at room temperature for 1 h. Themixture is extracted with 100 ml of dichloromethane, and the organicphase is washed with water, dried over Na₂SO₄, filtered and evaporated.The yellow crystals formed are dried at 160° C. and 10⁻² mbar for 5 min,giving a viscous, slowly crystallising oil.

Liquid-crystal mixtures having the compositions and properties asindicated in the following tables are prepared and investigated.

Examples 1.1 to 1.3 and Comparative Example 1

The following mixture (M-1), which comprises in total slightly more than20% of compounds of the formula III-O containing a CH₂O bridge, isprepared and investigated.

Mixture M-1 Composition Compound Concentration/ No. Abbreviation % byweight 1 COY-3-O1 <4.5 2 COY-3-O2 7.0 3 CCOY-2-1 3.0 4 CCOY-3-O2 9.0 5CPY-3-O2 5.5 6 CPY-3-O4 6.0 7 CPY-4-O3 5.5 8 CC-2-3 19.5 9 CC-3-4 10.010  CC-3-5 5.0 12  PP-1-5 12.5 13  CCP-3-1 8.0 14  CCP-3-3 4.5 Σ 100.0Physical properties T(N, I) = 76° C. Δn(20° C., 589 nm) = 0.0934 Δε(20°,1 kHz) = −2.2 γ₁(20° C.) = 100 mPa · s k₁₁(20° C.) = t.b.d. pN k₃₃(20°C.) = t.b.d. pN V₀(20° C.) = t.b.d. V Note: t.b.d.: to be determinedMixture M-1 is divided into five parts and investigated as describedbelow.

Comparative Example 1

Firstly, the stability of the voltage holding ratio of the mixture (M-1)itself is determined. Mixture M-1 itself is investigated in a test cellhaving an alignment material for homeotropic alignment and flat ITOelectrodes for their stability to lighting with a cold cathode((CCFL)-LCD backlight). To this end, corresponding test cells areexposed to the lighting for 1056 hours. The voltage holding ratio isthen determined in each case after 5 minutes at a temperature of 100° C.The results are summarised below in Table 1. Here, as below, six testcells are filled and investigated for each individual mixture. Thevalues indicated are the average of the six individual values and thestandard deviation (a) thereof, including for the case where thestandard deviation is smaller than the measurement value accuracyindicated above.

Examples 1.1 to 1.3

Next, alternatively 250 ppm of the compound of Synthesis Example 1 (i.e.of Substance Example 1)

or 250 ppm of the compound of Synthesis Example 2(i.e. of Substance Example 4)

or 250 ppm of the compound of Synthesis Example 7 (i.e. of SubstanceExample 50)

are added to mixture M-1, and the resultant mixtures (M-1-1 to M-1-3)are investigated for their stability as described above. The results areshown in the following table, Table 1.

The relative deviations of the “voltage holding ratio” values in variousmeasurement series are typically in the range up to about 2% and may inexceptional cases be up to a maximum of about 3 to 4%.

The drop in the voltage holding ratio (GVHR) usually caused by theexposure is determined as described in the text above.

TABLE 1 VHR(t)/% Ex. No. Mixture Stabilizer c(stab.)/ppm t = 0 h t =1.056 h V1 1 CM-1-0 none 0 94.6 ± 0.2 78 ± 1.3 B1.1 3 M-1-1 I(1)* 25096.4 ± 0.4 94 ± 0.9 B1.2 4 M-1-2 I(2)* 250 97.0 ± 0.3 92 ± 0.9 B1.3 4M-1-3 I(3)* 250 t.b.d. t.b.d. Notes: I(1)*: compound of SynthesisExample 1 I(2)*: compound of Synthesis Example 2 I(3)*: compound ofSynthesis Example 7 t.b.d.: to be determined

It can readily be seen here that the compound of Synthesis Example 1exhibits clearly stabilising properties. The starting mixture withoutthe use of the stabilizers exhibits a considerable drop in the VHR afterexposure by the backlight and is thus not suitable for use in mostmodern LCD applications. The values of the VHR after exposure of themixtures according to the invention which comprise the compounds of theformula I are superior both to those of the starting mixture and also tothose of the comparative mixture.

The VHR values before exposure by the backlight are significantlyimproved by all stabilizers used. The unstabilised reference exhibits asignificant drop in the VHR after exposure by the backlight. This can beprevented virtually completely by the stabilizers used. The best resultswere achieved with I(1). The use of the compounds indicated, inparticular the compounds of the formula I, thus also enables themixtures investigated to be used in modern, demanding LCD applications.

The compound of Synthesis Example 1 in a concentration of 250 ppm has astabilization activity which is superior to all other stabilizersinvestigated here. This results in a reduction in the occurrence ofimage sticking on exposure by the backlight.

Example 2 Examples 2.1 to 2.3 and Comparative Example 2

The following mixture (M-2), which comprises in total 13.5% of twocompounds of the formula III-O-2 containing a CH₂O bridge, is preparedand investigated.

Mixture M-2 Composition Compound Concentration/ No. Abbreviation % byweight 1 CCOY-2-O2 6.5 2 CCOY-3-O2 7.0 3 CY-3-O2 17.0 4 CPY-2-O2 12.0 5CPY-3-O2 11.0 6 CC-2-3 16.0 7 CC-3-4 10.0 8 CC-3-5 5.5 9 PP-1-3 4.5 10 PP-1-4 4.0 11  CCP-3-1 6.5 Σ 100.0 Physical properties T(N, I) = 75° C.n_(e)(20° C., 589 nm) = 1.5831 Δn(20° C., 589 nm) = 0.0997 ε_(⊥)(20°, 1kHz) = 6.7 Δε(20°, 1 kHz) = −3.3 γ₁(20° C.) = 112 mPa · s k₁₁(20° C.) =14.2 pN k₃₃(20° C.) = 14.5 pN V₀(20° C.) = 2.23 V

Mixture M-2 is, as described in Example 1, divided into four parts andtreated and investigated as described therein. The results are indicatedin the following table.

TABLE 2 VHR(t)/% Ex. No. Mixture Stabilizer c(stab.)/ppm t = 0 h t = 500h V2 1 M-2 none 0 95.7 ± 0.8 76.7 ± 1.5 B2.1 3 M-2-1 I(1)* 100 94.3 ±0.9 88.6 ± 1.3 B2.2 3 M-2-2 I(2)* 100 95.2 ± 0.3 91.5 ± 0.8 B2.3 3 M-2-3I(3)* 250 94.4 ± 0.7 91.7 ± 0.9 Notes: I(1)*: compound of SynthesisExample 1 I(2)*: compound of Synthesis Example 2 I(3)*: compound ofSynthesis Example 7

All compounds of the formula I used here significantly improve thestability of the mixtures used. This applies, in particular, to thevalues after the exposure test. (The values before the exposure test areall virtually indistinguishable here within the bounds of measurementaccuracy.) Stabilizer I(2) exhibits a particularly high activity here,even in a concentration of only 100 ppm, and results in a high VHR afterexposure by the backlight. Without the use of stabilizers, it was onlypossible to use mixture M-2 to a limited extent in an LCD. The use ofthe compounds of the formula I described causes an increase in the VHRto a value which enables use in most corresponding LCDs.

The compounds of Synthesis Example 2, even in a concentration of 100ppm, and those of Synthesis Example 7 in a concentration of 250 ppm havea superior stabilization activity to all other stabilizers investigatedhere. The compound of Synthesis Example 1 gives very good results evenin a concentration of only 100 ppm. The best absolute effect here isobserved for compounds I(2) and I(3), or, taking into account therespective concentrations employed, for compound I(2).

Example 3 Examples 3.1 to 3.3 and Comparative Example 3

The following mixture (M-3), which comprises 11% of a compound of theformula III-O-1 containing a CH₂O bridge, is prepared and investigated.

Mixture M-3 Composition Compound Concentration/ No. Abbreviation % byweight 1 COY-3-O2 11.0 2 CCY-3-O1 8.0 3 CCY-3-O2 11.0 4 CPY-2-O2 12.0 5CPY-3-O2 11.0 6 CC-2-3 20.0 7 CC-3-4 10.0 8 CC-3-5 6.0 9 PP-1-3 6.0 10 PP-1-4 5.0 Σ 100.0 Physical properties T(N, I) = 77° C. n_(e)(20° C.,589 nm) = 1.5844 Δn(20° C., 589 nm) = 0.1007 ε_(⊥)(20°, 1 kHz) = 6.5Δε(20°, 1 kHz) = −3.1 γ₁(20° C.) = 109 mPa · s k₁₁(20° C.) = 15.3 pNk₃₃(20° C.) = 14.5 pN V₀(20° C.) = 2.28 V

Mixture M-3 is, as described in Example 1, divided into four parts andtreated and investigated as described therein. The results aresummarised in the following table.

TABLE 3 VHR(t)/% Ex. No. Mixture Stabilizer c(stab.)/ppm t = 0 h t = 500h V3 1 M-3 none 0 96.2 ± 0.7 76.1 ± 2.1 B3.1 3 M-3-1 I(1)* 100 95.9 ±0.2 90.5 ± 0.5 B3.2 4 M-3-2 I(2)* 100 95.4 ± 0.6 90.8 ± 0.9 B3.3 5 M-3-3I(3)* 250 95.3 ± 0.6 91.7 ± 1.3 Notes: I(1)*: compound of SynthesisExample 1 I(2)*: compound of Synthesis Example 2 I(3)*: compound ofSynthesis Example 7

Mixture M-3 as such has a somewhat higher VHR, both before and afterexposure, than mixture M-2 of Example 2.

As in Example 2, all compounds of the formula I used here also have animproved VHR after exposure. Stabilizer I(3) exhibits a particularlyhigh activity here after exposure by the backlight. Without the use ofstabilizers, it was only possible to use mixture M-3 to a limited extentin an LCD. The use of the stabilizers described, in particular thecompounds of the formula I, causes an increase in the VHR to a valuewhich enables use in most corresponding LCDs.

Here too, as already in Example 2, the compounds of Synthesis Examples 1and 2, even in a concentration of 100 ppm, and the compound of SynthesisExample 7 in a concentration of 250 ppm have a significantly superiorstabilization activity to all other stabilizers investigated here. Thebest effect here is observed for compound I(3) of Synthesis Example 7employed in a concentration of 250 ppm.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

The invention claimed is:
 1. A liquid-crystalline medium comprising a)one or more compounds of formula I

in which n denotes 1, 2, 3 or 4, m denotes (4-n),

denotes an organic radical having 4 bonding sites, Z¹¹ and Z¹²,independently of one another, denote —O—, —(C═O)—, —(N—R¹⁴)— or a singlebond, but do not both simultaneously denote —O—, r and s, independentlyof one another, denote 0 or 1, Y¹¹ to Y¹⁴ each, independently of oneanother, denote alkyl having 1 to 4 C atoms and alternatively, alsoindependently of one another, one or both of the pairs (Y¹¹ and Y¹²) and(Y¹³ and Y¹⁴) together denote a divalent group having 3 to 6 C atoms,R¹¹ denotes O—R¹³, O^(•) or OH, R¹² on each occurrence, independently ofone another, denotes H, F, OR¹⁴, NR¹⁴R¹⁵, a straight-chain or branchedalkyl chain having 1-20 C atoms, in which one —CH₂— group or a pluralityof —CH₂— groups may be replaced by —O— or —C(═O)—, but two adjacent—CH₂— groups cannot be replaced by —O—, or denotes a hydrocarbon radicalwhich contains a cycloalkyl or alkylcycloalkyl unit, and in which one—CH₂— group or a plurality of —CH₂— groups may be replaced by —O— or—C(═O)—, but two adjacent —CH₂— groups cannot be replaced by —O—, and inwhich one H atom or a plurality of H atoms may be replaced by OR¹⁴,N(R¹⁴)(R¹⁵) or R¹⁶, or denotes an aromatic or heteroaromatic hydrocarbonradical, in which one H atom or a plurality of H atoms may be replacedby OR¹⁴, N(R¹⁴)(R¹⁵) or R¹⁶, R¹³ on each occurrence, independently ofone another, denotes a straight-chain or branched alkyl chain having1-20 C atoms, in which one —CH₂— group or a plurality of —CH₂— groupsmay be replaced by —O— or —C(═O)—, but two adjacent —CH₂— groups cannotbe replaced by —O—, or denotes a hydrocarbon radical which contains acycloalkyl or alkylcycloalkyl unit, and in which one —CH₂— group or aplurality of —CH₂— groups may be replaced by —O— or —C(═O)—, but twoadjacent —CH₂— groups cannot be replaced by —O—, and in which one H atomor a plurality of H atoms may be replaced by OR¹⁴, N(R¹⁴)(R¹⁵) or R¹⁶,or denotes an aromatic or heteroaromatic hydrocarbon radical, in whichone H atom or a plurality of H atoms may be replaced by OR¹⁴,N(R¹⁴)(R¹⁵) or R¹⁶, or can be

(1,4-cyclohexylene), in which one or more —CH₂— groups may be replacedby —O—, —CO— or —NR¹⁴—, or an acetophenyl, isopropyl or 3-heptylradical, R¹⁴ on each occurrence, independently of one another, denotes astraight-chain or branched alkyl or acyl group having 1 to 10 C atoms oran aromatic hydrocarbon or carboxyl radical having 6-12 C atoms, R¹⁵ oneach occurrence, independently of one another, denotes a straight-chainor branched alkyl or acyl group having 1 to 10 C atoms or an aromatichydrocarbon or carboxyl radical having 6-12 C atoms, R¹⁶ on eachoccurrence, independently of one another, denotes a straight-chain orbranched alkyl group having 1 to 10 C atoms, in which one —CH₂— group ora plurality of —CH₂— groups may be replaced by —O— or —C(═O)—, but twoadjacent —CH₂— groups cannot be replaced by —O—, with the provisos that,in the case where n=1, R¹¹=O^(•) and —[Z¹¹-]_(r)-[Z¹²]_(s)—=—O—,—(CO)—O—, —O—(CO)—, —O—(CO)—O—, —NR¹⁴— or —NR¹⁴—(CO)—,

does not denote a straight-chain or branched alkyl having 1 to 10 Catoms, cycloalkyl, cycloalkylalkyl or alkylcycloalkyl, and in the casewhere n=2 and R¹¹=O—R¹³, R¹³ does not denote n-C₁₋₉-alkyl, and Compoundsof the formula

are excluded from formula 1, b) one or more compounds of the formulaIII-O

in which R³¹ denotes an unsubstituted alkyl radical having 1 to 7 Catoms, R³² denotes an unsubstituted alkyl radical having 1 to 7 C atomsor an unsubstituted alkoxy radical having 1 to 6 C atoms, and i denotes0 or
 1. 2. The medium according to claim 1, comprising one or morecompounds of the formula I that have formulae I-1 to I-9

in which t denotes an integer from 1 to 12, R¹⁷ denotes a straight-chainor branched alkyl chain having 1-12 C atoms, in which one —CH₂— group ora plurality of —CH₂— groups may be replaced by —O— or —C(═O)—, but twoadjacent —CH₂— groups cannot be replaced by —O—, or denotes an aromaticor heteroaromatic hydrocarbon radical, in which one H atom or aplurality of H atoms may be replaced by OR¹⁴, N(R¹⁴)(R¹⁵) or R¹⁶.
 3. Themedium according to claim 1, comprising one or more compounds of theformula I that have the formulae I-1a-1 to I-8a-1


4. The medium according to claim 1, additionally comprising c) one ormore compounds of formula II

in which R²¹ denotes an unsubstituted alkyl radical having 1 to 7 Catoms or an unsubstituted alkenyl radical having 2 to 7 C atoms, and R²²denotes an unsubstituted alkenyl radical having 2 to 7 C atoms, and/ord) one or more compounds of formulae III-1 to III-4

in which R³¹ denotes an unsubstituted alkyl radical having 1 to 7 Catoms, R³² denotes an unsubstituted alkyl radical having 1 to 7 C atomsor an unsubstituted alkoxy radical having 1 to 6 C atoms, and m, n and oeach, independently of one another, denote 0 or
 1. 5. The mediumaccording to claim 1, having a total concentration of the compounds ofthe formula I in the medium as a whole of 1 ppm to 1000 ppm.
 6. Themedium according to claim 4, comprising a compound of the formula II, inwhich R²¹ denotes n-propyl and R²² denotes vinyl.
 7. The mediumaccording to claim 1, having a total concentration of the compounds ofthe formula III-O in the medium as a whole of 5% to 40%.
 8. The mediumaccording to claim 1, comprising one or more compounds of formulaIII-O-1

in which R³¹ and R³² have the respective meanings given for formulaIII-O.
 9. The medium according to claim 1, comprising one or morecompounds of formula III-O-2

in which R³¹ and R³² have the respective meanings given for formulaIII-O.
 10. The medium according to claim 1, additionally comprising oneor more chiral compounds.
 11. An electro-optical display orelectro-optical component containing a liquid-crystalline mediumaccording to claim
 1. 12. The display according to claim 11, operatingby VA or ECB effect.
 13. The display according to claim 11, containingan active-matrix addressing device.
 14. A process for the preparation ofa liquid-crystalline medium according to claim 1, comprising mixing oneor more compounds of formula I with one or more compounds of formulaIII-O and/or one or more compounds of formula II and/or one or morecompounds of formulae III-1 to III-4;

in which R²¹ denotes an unsubstituted alkyl radical having 1 to 7 Catoms or an unsubstituted alkenyl radical having 2 to 7 C atoms, and R²³denotes an unsubstituted alkenyl radical having 2 to 7 C atoms;

wherein m1, n1 and o are each independently 0 or
 1. 15. A method for thestabilization of a liquid-crystalline medium according to claim 1,comprising adding to the medium one or more compounds of formula III-O,and one or more compounds of formula I and optionally one or morecompounds of formulae OH-1 to OH-6